ICU library symbols not found while linking on Kubuntu 20.04

Compiling Qt 6Beta5 QtCore fails on ICU libraries. The symbols are suffixed with _66, which is the version I have installed, but linking fails.

apt-get install -y libicu66 libicu-dev

libicu-dev is already the newest version (66.1-2ubuntu2).
libicu66 is already the newest version (66.1-2ubuntu2).

For example, I am seeing messages as this:

/usr/bin/ld: qtbase/src/corelib/CMakeFiles/Core.dir/text/qcollator_icu.cpp.o: in function `QCollatorPrivate::cleanup()':
qcollator_icu.cpp:(.text+0x12): undefined reference to `ucol_close_66'

Oddly enough, the attached configure log qtconfig.log shows the installed ICU libraries have been found, but it seems the symbols are not named as expected.

Config option -icu is apparently regarded by configure "boolean", so I cannot explicitly configure the build to make use of the libraries I have installed (e.g. --icu=system) nor does disabling it (e.g. -no-icu, as was the case here) make any difference.

If you search the bug database for bugs with the text ICU, you will find that historically there have been problems with finding the ICU libraries included with Qt.

Files:
config.txt: The config options I used. Lines prepended with '#' are comments.
qtconfig.log: Output from the configuration
qtbuild.log: Output from the build process

Recommendation:
Enable the user to configure the build to use the installed ICU libraries.